The present invention is in the field of battery technology and, more particularly, in the area of solid polymeric materials and composites for use in electrodes and electrolytes in electrochemical cells.
Conventional lithium ion batteries include a positive electrode (or cathode as used herein), a negative electrode (or anode as used herein), an electrolyte, and, frequently, a separator. The electrolyte typically includes a liquid component that facilitates lithium ion transport and, in particular, enables ion penetration into the electrode materials.
In contrast, so-called solid-state lithium ion batteries do not include liquid in their principal battery components. Solid-state batteries can have certain advantages over liquid electrolyte batteries, such as improvements in safety because the liquids used in liquid electrolytes are often volatile organic solvents. Solid-state batteries offer a wider range of packaging configurations because a liquid-tight seal is not necessary as it is with liquid electrolytes.
Further, solid state batteries can use lithium metal as the anode, thereby dramatically increasing the energy density of the battery as compared to the carbon-based anodes typically used in liquid electrolyte lithium ion batteries. With repeated cycling, lithium metal can form dendrites, which can penetrate a conventional porous separator and result in electrical shorting and runaway thermal reactions. This risk is mitigated through the use of a solid nonporous polymer electrolyte.
The electrolyte material in a solid-state lithium ion battery can be a polymer. In particular, poly(ethylene oxide) (“PEO”) can be used in forming solid polymer electrolytes. PEO has the ability to conduct lithium ions as positive lithium ions are solubilized and/or complexed by the ethylene oxide groups on the polymer chain. Solid electrolytes formed from PEO can have crystalline and amorphous regions, and it is believed that lithium ions move preferentially through the amorphous portion of the PEO material. In general, ionic conductivities on the order of 1×10−6 S/cm to 1×10−5 S/cm at room temperature can be obtained with variations on PEO based electrolyte formulations. The electrolyte is typically formulated by adding a lithium ion salt to the PEO in advance of building the battery, which is a formulation process similar to liquid electrolytes.
However, solid-state batteries have not achieved widespread adoption because of practical limitations. For example, while polymeric solid-state electrolyte materials like PEO are capable of conducting lithium ions, their ionic conductivities are inadequate for practical power performance. Successful solid-state batteries require thin film structures, which reduce energy density, and thus have limited utility.
Further, solid-state batteries tend to have a substantial amount or degrees of interfaces among the different solid components of the battery. The presence of such interfaces can limit lithium ion transport and impede battery performance. Interfaces can occur (i) between the domains of active material in the electrode and the polymeric binder, (ii) between the cathode and the solid electrolyte, and (iii) between the solid electrolyte and the anode structure. Poor lithium ion transport across these interfaces results in high impedance in batteries and a low capacity on charge or discharge.
Research on solid-state electrolyte materials tends to focus primarily on the composition of the materials used to form the electrolyte to increase ion conductivity. However, less attention has been paid to solving the problem of increased impedance due to conductivity losses at interfaces or addressing the transport of ions through the electrode structures.
For example, U.S. Patent Publication 2013/0026409 discloses a composite solid electrolyte with a glass or glass-ceramic inclusion and an ionically conductive polymer. However, this solid electrolyte requires a redox active additive. As another example, U.S. Pat. No. 5,599,355 discloses a method of forming a composite solid electrolyte with a polymer, salt, and an inorganic particle (such as alumina). The particles are reinforcing filler for solid electrolyte and do not transport lithium. As yet another example, U.S. Pat. No. 5,599,355 discloses a composite solid state electrolyte containing a triflate salt, PEO, and a lightweight oxide filler material. Again, the oxide filler is not a lithium ion conductor or intercalation compound.
More generally, ionically conductive polymers like PEO have been disclosed with the use of a lithium salt as the source of lithium ions in the solid electrolyte. For example, Teran et al., Solid State Ionics (2011) 18-21; Sumathipala et al., Ionics (2007) 13: 281-286; Abouimrane et al., JECS 154(11) A1031-A1034 (2007); Wang et al., JECS, 149(8) A967-A972 (2002); and Egashira et al., Electrochimica Acta 52 (2006) 1082-1086 each disclose different solid electrolyte formulations with PEO and a lithium salt as the source for lithium ions. Still further the last two references (Wang et al. and Egashira et al.) each disclose inorganic nanoparticles that are believed to improve the ionic conductivity of the PEO film by preventing/disrupting polymer crystallinity. However, none of these formulations address the limitations of solid electrolytes and provide the performance improvements seen in the embodiments disclosed below.